A boost converter, conventionally known as one of power semiconductor devices, boosts an input voltage to a desired voltage by using a semiconductor switching element such as an MOSFET (metal oxide semiconductor field effect transistor) and a driving IC (integrated circuit) that drives the semiconductor switching element. Generally, an AC voltage such as that supplied from a household AC power source or applied across an auxiliary winding is converted by a rectifier smoothing circuit composed of a diode, a capacitor and the like to a DC voltage, and the DC voltage is supplied to the boost converter as a power source voltage to drive the boost converter, as described for example in Japanese Patent Application Laid-Open Nos. 2012-74829 and 2006-53803, for example.
The power source voltage being obtained by rectifying the AC voltage and being supplied to the boost converter suffers from a problem such as its instability. Hence, in order to stabilize the power source voltage to drive the boost converter, an IC chip may be given a built-in linear regulator function capable of outputting a fixed voltage. However, a linear regulator circuit generally releases heat generated by a difference between an input voltage input to the linear regulator circuit and an output voltage output from the linear regulator circuit. Thus, a larger amount of heat is released in response to the increase of this difference.
This has made it difficult to provide an IC chip with a built-in linear regulator function that generates a power source voltage of about 15 V to drive the boost converter from a relatively high input voltage of about 100 V or more obtained by rectifying a commercial AC power source for example, in light of a problem such as temperature increase caused by heat generation by the linear regulator function.